Department of Physics, University of California, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 2013 Apr 23;110(17):6688-93. doi: 10.1073/pnas.1217553110. Epub 2013 Apr 8.
The magnetic fields of Earth and other planets are generated by turbulent, rotating convection in liquid metal. Liquid metals are peculiar in that they diffuse heat more readily than momentum, quantified by their small Prandtl numbers, Pr << 1. Most analog models of planetary dynamos, however, use moderate Pr fluids, and the systematic influence of reducing Pr is not well understood. We perform rotating Rayleigh-Bénard convection experiments in the liquid metal gallium (Pr = 0.025) over a range of nondimensional buoyancy forcing (Ra) and rotation periods (E). Our primary diagnostic is the efficiency of convective heat transfer (Nu). In general, we find that the convective behavior of liquid metal differs substantially from that of moderate Pr fluids, such as water. In particular, a transition between rotationally constrained and weakly rotating turbulent states is identified, and this transition differs substantially from that observed in moderate Pr fluids. This difference, we hypothesize, may explain the different classes of magnetic fields observed on the Gas and Ice Giant planets, whose dynamo regions consist of Pr < 1 and Pr > 1 fluids, respectively.
地球和其他行星的磁场是由液态金属中湍流、旋转的对流产生的。液态金属的奇特之处在于，它们比动量更容易扩散热量，这由它们的小普朗特数 Pr 来量化，Pr << 1。然而，大多数行星发电机的模拟模型都使用中等 Pr 的流体，并且降低 Pr 的系统影响还没有得到很好的理解。我们在液态金属镓（Pr = 0.025）中进行了一系列无量纲浮力强迫（Ra）和旋转周期（E）的旋转瑞利-贝纳德对流实验。我们的主要诊断是对流热传递效率（Nu）。总的来说，我们发现液态金属的对流行为与中等 Pr 流体（如水）有很大的不同。特别是，我们确定了旋转约束和弱旋转湍流状态之间的转变，并且这个转变与在中等 Pr 流体中观察到的转变有很大的不同。我们假设，这种差异可能解释了在气体和冰巨行星上观察到的不同类型的磁场，它们的发电机区域分别由 Pr < 1 和 Pr > 1 的流体组成。
